1. Field of the Invention
Urea granules or prills which have been coated with sulfur to slow the rate of dissolution of the urea in the soil when they are applied as a fertilizer can be coated with a sealant to further slow the release. This application relates to improved sealant compositions which provide superior moisture barrier properties and are non-blocking and thus do not require the addition of clay or other fine particle materials.
2. Description of the Prior Art
The process for sulfur coating of urea granules or prills was developed in 1968 by the Tennessee Valley Authority (TVA) in Muscle Shoals, Ala., as an economical system for reducing the rate of dissolution of urea particles when they are applied to the soil as fertilizer. U.S. Pat. No. 3,342,577 describes this sulfur coating process and also the sealant material that was necessary to fill in the holes that result naturally in the sulfur coating as it cools. The TVA process is also described in Sulphur Inst. J. 4(3) 2-6 (1968), ibid. 8(4) 2-5 (1972a) and ibid. 8(4) 6-8 (1972b). The TVA recommendation for the sealant now used by the majority of manufacturers of sulfur coated urea is a mixture of 30% polyethylene resin in 70% bright stock mineral oil.
The process is further described in Sulfur Coating of Urea Treated with Atapulgite Clay, Gullett, G. L.; Simons, C. L.; and Lee, R. G.; presented at the 198th American Chemical Society meeting in Miami Beach, Fla., in September 1989.
The requirement for a sealant for sulfur coated urea (SCU) has been documented by McClellan and Scheib (Sulphur Inst. J. 9(3/4) 8-12 (1973), and by Scheib and McClellan ibid. 12(1) 2-5 (1976).
U.S. Pat. No. 4,042,366 describes SCU coated with a wax in which a water soluble additive is embedded. The wax used on the SCU by itself without the additive particles produces fertilizer particles which are tacky and not free flowing; the water soluble additive mitigates the tack, in a manner similar to the clay which is used in the TVA process.
Japanese patent 56-69295 discloses polymer resins used over SCU which provide free flowing particles. The resins are viscous and high melting and must be applied from a solvent solution. The solvent is then removed, creating either severe environmental and safety problems or great expense to prevent the release of the solvents to the workplace or the environment.
A description of slow release urea and NPK fertilizers is given in Hort. Rev. 1 79-140 (1979).
Paraffin waxes have been used to produce relatively slow dissolving clathrate complexes with urea by processes which do not relate to coating urea prills, but instead involve a solution or dispersion of urea in paraffin, as described in U.S. Pat. No. 3,252,786.
Paraffin has not been used as a slow release coating for urea because of its lack of adhesion. Also paraffin has not been used as a sealant for SCU for the same reason and also because when the SCU prills or granules are handled in high speed bulk moving equipment wherein large amounts of product are moved by hopper car or by truck and off-loaded into storage bins, the coating is rubbed off or otherwise cracked or abraded.
The wax-oil sealants as described by TVA publications and currently being applied to SCU require the addition of a clay conditioning agent at levels nearly equal to that of the wax sealant to prevent caking and provide a free flowing product. A typical process may require 3% of the wax-oil sealant and 2% of the clay conditioning agent on the weight of the SCU, or 67% clay on the weight of the sealant. This requires large scale clay handling equipment in addition to the equipment necessary to apply the molten sulfur and wax sealant.
When the clay conditioned SCU is applied in the field by mechanical spreaders, the clay or wax-clay mixtures tend to be removed from the SCU particles by the abrasive action of the screw conveyors and the mechanical spreading wheel, resulting in a build-up of wax-clay on various parts of the machine and requiring frequent shut-downs for cleaning.